1. Field of the Invention
The present invention relates to a method of manufacturing a Ni-based superalloy component for a gas turbine using a one-step process of hot isostatic pressing (HIP) and heat treatment, and a component manufactured thereby and, more particularly, to a method of manufacturing a Ni-based superalloy component for a gas turbine using a one-step process of HIP and heat treatment, in which an HIP process and a heat treatment process, which have been typically separately performed to manufacture or repair the Ni-based superalloy component for a gas turbine, are performed as a one-step process using an HIP apparatus, and a component manufactured by the method. Thus, component defects, such as micropores or microcracks, which are generated when casting, welding, or brazing the Ni-based superalloy component for a gas turbine used for a combined cycle thermal power plant or airplane, can be cured using an HIP apparatus at a high temperature and, at the same time, the physical properties of the Ni-based superalloy component can be optimized using the heat treatment process.
2. Description of the Related Art
Most hot gas components used for gas turbines that directly run on gas generated by burning fossil fuels are formed of Ni-based superalloy materials (e.g., GTD-111) by a precision casting process. When a hot gas component used for a predetermined time is slightly damaged, the damaged portions are repaired using a welding or brazing method to reuse the hot gas component. However, since the repaired hot gas component may not be perfect due to cast defects (e.g., micropores) or welding cracks, these defects are cured using a hot isostatic pressing (HIP) apparatus to densify the structure of the hot gas component.
Also, the hot gas components for gas turbines, which are cast or repaired using a welding or brazing method, are subjected to a series of heat treatments in order to optimize physical properties (e.g., high-temperature tensile strength and creep resistance characteristics) of the Ni-based superalloy material. For example, Rene 80, which is a Ni-based superalloy, undergoes the following four-step heat treatment.
[Step 1] Rene 80 is vacuum processed at a temperature of about 2175 to 2225° F. (about 1191 to 1218° C.) for 2 hours, and then furnace-cooled in a vacuum atmosphere or in an Ar or He atmosphere to a temperature of about 1975 to 2025° F. (about 1079 to 1107° C.) within 10 minutes.
[Step 2] Rene 80 is vacuum processed at a temperature of about 1975 to 2025° F. (about 1079 to 1107° C.) for 4 hours, furnace-cooled in a vacuum atmosphere or in an Ar or He atmosphere to a temperature of about 1200° F. (about 649° C.) within 60 minutes, and then maintained at a temperature of about 1200° F. (about 649° C.) for 10 minutes.
[Step 3] Rene 80 is heated in a vacuum atmosphere to a temperature of about 1925° F. (about 1052° C.), maintained in a vacuum atmosphere or in an Ar or He atmosphere at a temperature of about 1900 to 1950° F. (about 1038 to 1066° C.) for 2 to 12 hours, cooled to a temperature of about 1200° F. (about 649° C.) within 15 to 60 minutes, and then maintained for 10 minutes.
[Step 4] Rene 80 is heated in a vacuum atmosphere or in an Ar or He atmosphere to a temperature of about 1550° F. (about 843° C.), maintained at a temperature of about 1525 to 1575° F. (about 829 to 857° C.) for 16 hours, and then furnace-cooled or air-cooled to a room temperature.
The foregoing heat treatment is carried out to control high-temperature physical properties of the Ni-based superalloy material, especially the shape and size of a gamma prime phase which is a high-temperature precipitation phase.
FIGS. 1A and 1B are scanning electron microscope (SEM) photographs showing dendritic and interdendritic microstructures of Ni-based superalloy components treated by a conventional heat treatment. Referring to FIG. 1A, it can be seen that square-shaped gamma-prime phases with a size of about 0.4 μm are uniformly distributed. The dendritic microstructures shown in FIG. 1A are obtained by processing the cast Ni-based superalloy using the conventional heat treatment.
Meanwhile, an HIP process is a commercially available process that is simply performed at predetermined temperature and pressure (e.g., 1190° C. and 100 MPa) for several hours. The microstructures of a material processed using the HIP process are quite different from those shown in FIGS. 1A and 1B.
For example, after a component obtained by casting and heat treating a Ni-based superalloy is used over a long period, the gamma prime phases become slightly rounded as shown in FIG. 2A. Although the gamma prime phases are changed into microstructures as shown in FIG. 2B when treated by the commercially available HIP process, they are quite different from the microstructures shown in FIGS. 1A and 1B.
Accordingly, in order to optimize the physical properties of the Ni-based superalloy even after performing the HIP process, the Ni-based superalloy is subjected to an additional heat treatment to obtain the microstructures shown in FIGS. 1A and 1B, in the same manner as when the Ni-based superalloy is cast. Accordingly, the conventional, commercially practiced, HIP process and heat treatment are separately performed. In most cases, the HIP process is followed by the heat treatment.
Conventionally, the time and manpower required to manufacture the hot gas components increase due to the two-step process including the HIP process and the heat treatment. Moreover, the unit cost of products increases since separate equipment is required for the HIP process and the heat treatment. Furthermore, the process of manufacturing or repairing the component is extended, thus increasing the failure rate.